Creating more concern at the global level, avian influenza, or bird flu, is on the rise in the United States, with public health officials on alert after an unprecedented spread in dairy cows this year. According to the United States Centers for Disease Control and Prevention (CDC), four dairy workers have also tested positive in the country. The first-ever known case of cow-to-human transmission of bird flu viruses in the United States and globally was reported on April 1, 2024, and it was the second-ever documented human case of bird flu in the United States.
The first U.S. case of H5N1 bird flu was reported in a poultry worker in Colorado in 2022. The initial two patients in this outbreak presented with conjunctivitis as their chief complaint, while the third patient exhibited more typical flu symptoms, including a cough. All three had direct contact with infected cows. On July 3, 2024, a fourth human case linked to the dairy cow outbreak was identified in Colorado, with this patient reporting only eye symptoms.
Since 2020, a particularly severe variant of the H5N1 strain has been spreading globally among animals, causing deadly outbreaks in commercial poultry and sporadic infections in various species, from alpacas to house cats. Until this year, cows had not been affected.
Different bird flu strains have been detected in humans in Australia and Mexico, while various H5 subtypes are present in both animals and humans in countries such as China and Cambodia. Most human cases involved exposure to poultry, live poultry markets, or dairy cattle before infection. However, scientists are concerned that the virus could mutate to spread more easily from person to person, potentially sparking a pandemic. According to the World Health Organization, the risk to humans remains low at this time.
Meanwhile, the first known cases of infected dairy cattle occurred in Texas in March and have now spread to dairy herds in 12 states. The U.S. Department of Agriculture reported that tests indicate the virus detected in cows is the same H5N1 strain affecting wild birds and commercial poultry flocks. The four dairy workers who tested positive for the virus this year experienced mild symptoms such as conjunctivitis, or pink eye.
The H5N1 virus in the United States belongs to clade 2.3.4.4b, genotype B3.13, a genotype detected only in North America so far, according to a scientific report by the European Food Safety Agency.
